1. Field of the Invention
The invention relates to the enterprise telecommunication networks. An enterprise telecommunication network often links terminals that are spread on a plurality of sites, and today it uses the Internet Protocol (IP) and the Session Initiation Protocol (SIP). Voice samples, data packets, and signaling messages are carried by an IP network that is independent of the public telephone switched network (PSTN), though the enterprise network is also linked to the PSTN by a gateway in order to communicate with the world outside the enterprise.
Each SIP terminal runs a SIP User Agent. All the sip user agents are interconnected through an IP network comprising at least one SIP server that acts as a registrar server, a redirect server, a proxy server, and possibly as a presence server, in order to set up telephone communications, and also provide numerous telephony services. For instance: message waiting indications, presence state indications, conference membership notifications, call statistics, messaging . . . . Generally an enterprise network comprises a single SIP server, located in a main site of the enterprise, and the terminals of the remote sites communicate with the SIP server of the main site via an IP network.
In case of failure of this IP network, part or totality of the services previously provided by a SIP server in the main site is no longer available in the remote sites.
2. Description of the Prior Art
Some solutions are known:                Calls may be rerouted to the PSTN but none out of band service information is conveyed through this way.        Some local proxy servers may offer specific services in case of loss of the link with the SIP server of the main site, but they do not exchange any information with it as long as the IP network is down; so, all the services hosted in the SIP server of the main site are no longer available in the remote site.        Global IP routing through the PSTN, by means of a fully backed up infrastructure. The cost of deployment and the cost of the required bandwidth are very high.        
The aim of the present invention is to provide at least some of these telephony services to the SIP terminals located on remote sites, when the IP link to the SIP server of the main site is down, at a reasonable cost.